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Acids and Alkalis

What is an Indicator?

An indicator is a substance that changes colour at
the end point of a titration. An indicator is added
to the solution in the conical flask (see an example).

Different indicators are used depending on
the strength of the acid or alkali.
The indicator used for a particular titration is given in the table below.

Indicators used for weak Acids or Alkalis.

Indicator Titration Colour in Acid Colour in Alkali
Methyl Orange Strong Acid
Weak Alkali
Red Yellow
Phenolphthalein Strong Alkali
Weak Acid
Colourless Pink

Either Indicator can be used for a Strong Alkali + Strong Acid.

Universal indicator is not usually used for a titration because
it changes gradually giving different colours for a different pH.
Methyl orange or phenolphthalein are used
because they give a sudden change in colour at neutralisation
which makes it easier to see the end point of the titration.

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